1. Technical Field
The present invention relates to ferroelectric capacitors and methods for manufacturing the same.
2. Related Art
Ferroelectric memory devices (FeRAM) are nonvolatile memory devices capable of low voltage and high-speed operation, using spontaneous polarization of the ferroelectric material, and their memory cells can be each formed from one transistor and one capacitor (1T/1C). Accordingly, ferroelectric memory devices can achieve integration at the same level of that of DRAM, and are therefore expected as large-capacity nonvolatile memories.
As the material for forming the ferroelectric film that composes the ferroelectric capacitor, in other words, as the ferroelectric material, a material having a perovskite type crystal structure expressed by a general formula ABO3, more specifically, lead zirconate titanate (Pb (Zi, Ti) O3: PZT) may generally be used.
For the next generation ferroelectric memories, it is desired to form ferroelectric films by a MOCVD (metal organic chemical vapor deposition) method. This is because the conventional sputter method cannot form ferroelectric films with satisfactory crystal orientation characteristic, and good ferroelectric characteristic cannot be obtained, while ferroelectric films with good crystal orientation characteristic are desired.
Japanese Laid-open Patent Application JP-A-2003-21835 proposes a method as a technology to meet such demands. This technology pertains to a method in which a first ferroelectric film in amorphous phase is crystallized, and then a second ferroelectric film in crystalline phase is deposited on the top surface of the first ferroelectric film, wherein the second ferroelectric film is deposited and formed by a MOCVD method.
However, the inventors have diligently studied ferroelectric films formed by the aforementioned MOCVD method, and discovered that ferroelectric capacitors obtained through forming a film of PZT by the MOCVD method to form a ferroelectric film had defects with considerable leakage current. Although the cause of this considerable leakage current is not clear, it is believed that this may be caused by roughened morphology of the PZT film formed by the MOCVD method. It is believed that the morphology roughness may be generated due to the formation of roughness in the surface of the PZT film because partially heterogeneous crystal surfaces are exposed in the surface of the PZT film formed by the MOCVD method, and the portions with the exposed heterogeneous crystal surfaces form protrusions.